Fiber optic cables are often connected together by aligning and pressing the ends of two fibers together. The end of the fibers (the ‘end-faces’) are typically polished smooth and flat, or at an angle. The optical coupling occurs between the cores of the fibers, which is the central portion of the fiber that guides the optical energy. The types of fiber can be single-mode-fiber (SMF), with a core that is usually 9 microns in diameter, or multi-mode-fiber (MMF), with a core that is much larger, but typically between 50 to 100 microns in diameter. Efficient optical coupling occurs when the cores of the two fibers are aligned and in physical contact. Ideally, nearly 100% of the light is coupled between the two fibers, but in practice, a loss of up to 0.3 dB may be acceptable.
Imperfections in the fiber end-face polished surface or contamination trapped between the cores of the fibers can reduce the efficiency of the optical coupling. These imperfections can also create an increased amount of back-reflected light from the connector interface. Imperfections can arise during the handling and use of the fiber. Imperfections can be in the form of scratches or other mechanical damage to the end-face of the fiber. Contamination can result from liquid sources or oils on the fiber end-face. Contamination can also result from particles trapped within the fiber-to-fiber interface. Particles can originate from the connector itself, for example, from the regions where the mechanical alignment mechanisms engage (such as guide holes), or from external sources, such as dust in the environment outside the connector. A trapped particle can further damage the end-face polish if the particle hardness is similar or greater that the glass in the fiber core. A particle can create scratches on the fiber end-face.
The optical coupling efficiency between the two fiber cores is reduced if the fiber cores are not in physical contact and an air gap is created between the cores. An air gap will create a Fresnel reflection of approximately 4% at each of the two core-to-air interfaces, a double Fresnel reflection. If this light is coherent, the interference of the reflections can create additional coupling loss.
Multi-fiber connectors are designed to bring two arrays of fiber end-faces into alignment and create physical contact between the fiber cores. The manufacturing process typically polishes the fiber connector end-face, polishing multiple fibers simultaneously. The polishing process typically leaves the tips of the fibers slightly protruding from the connector face by 1 to 3 microns. This allows two connectors to mate and have the fiber end-faces make physical contact.
The protrusions of the fiber tips on the connector are not typically perfectly uniform. The polishing process may leave a taper or a curvature across the array. Therefore, there is a provision in the connector to allow the fibers to recess under pressure. A spring can be provided within the connector to create the pressure. As two fiber connectors mate, the fibers that have a greater protrusion will come into contact first. Under pressure, these two fibers will recede into their connector until fibers with less protrusion make physical contact.
A failure in the recess mechanism may make a fiber fail to rebound after it has been recessed. This failure is called ‘pistoning’. The fiber tip has been pressed down into the connector, but does not restore to a protruding state after un-mating of the connector. Pistoning can cause failure of a subsequent mating, as the fiber is not protruding enough to create physical contact.
Damage may occur to the fiber end-face during the process of manufacturing the fiber optic cable. There may be steps of handling the cable for testing, inspection or installation of the cable into a higher-level assembly. The manufacturer may ship the cable to a customer that further handles the cable before final installation into a network.
Fiber optics are finding use in applications that operate in harsh environments, such as aircraft, helicopters, unmanned vehicles, ship-board, space-craft and missiles. The fiber optic components must be able to operate and survive in an environment with severe shock, vibration, exposure to liquid contaminates, and over wide temperature ranges (such a −55 C to 125 C). These environmental stresses can cause the fiber end-faces, in physical contact within a connector, to become damaged or contaminated. Damage may occur when a particle trapped in the optical interface is moved along the fiber end-face due to vibration, shock or thermal expansion/contraction. This movement may leave scratches on the polish surface of the fiber end-face. An environment that exposes the connector to liquid contaminate can compromise optical coupling if the liquid enters into an air gap between two fiber cores.
Therefore, there has been a long-standing need for systems and methods for providing more precise fiber end coupling. Details of such systems and methods are provided below.